Ivan Lisenkov | Oregon State University (original) (raw)

Papers by Ivan Lisenkov

It is demonstrated theoretically that a thin layer of an anisotropic antiferromagnetic (AFM) insu... more It is demonstrated theoretically that a thin layer of an anisotropic antiferromagnetic (AFM) insulator can effectively conduct spin current through the excitation of a pair of evanescent AFM spin wave modes. The spin current flowing through the AFM is not conserved due to the interaction between the excited AFM modes and the AFM lattice, and, depending on the excitation conditions, can be either attenuated or enhanced. When the phase difference between the excited evanescent modes is close to π/2, there is an optimum AFM thickness for which the output spin current reaches a maximum, that can significantly exceed the magnitude of the input spin current. The spin current transfer through the AFM depends on the ambient temperature and increases substantially when temperature approaches the Neel temperature of the AFM layer. Progress in modern spintronics critically depends on finding novel media that can serve as effective conduits of spin angular momentum over large distances with minimum losses [1–3]. The mechanism of spin transfer is reasonably well-understood in ferromagnetic (FM) metals [4, 5] and insulators [3, 4, 6–9], but there are only very few theoretical papers describing spin current in an-tiferromagnets (AFM) (see, e.g., [10]). The recent experiments [11–13] have demonstrated that a thin layer of a dielectric AFM (NiO, CoO) could effectively conduct spin current. The transfer of spin current was studied in the FM/AFM/Pt trilayer structure (see Fig. 1). The FM layer driven in ferromagnetic resonance (FMR) excited spin current in a thin layer of AFM, which was detected in the adjacent Pt film using the inverse spin Hall effect (ISHE). It was also found in [13] that the spin current through the AFM depends on the ambient temperature and goes through a maximum near the Neel temperature T N. The most intriguing feature of the experiments was the fact that for a certain optimum thickness of the AFM layer (∼ 5 nm) the detected spin current had a maximum [11, 12], which could be even higher than in the absence of the AFM spacer [12]. The spin current transfer in the reversed geometry, when the spin current flows from the Pt layer driven by DC current through the AFM spacer into a microwave-driven FM material has been reported recently in [14]. The experiments [11–14] posed a fundamental question of the mechanism of the apparently rather effective spin current transfer through an AFM dielectric. A possible mechanism of the spin transfer through an easy-axis AFM has been recently proposed in [10]. However, this uniaxial model can not explain the non-monotonous dependence of the transmitted spin current on the AFM layer thickness and the apparent " amplification " of the spin current seen in the experiments [11, 12] performed with the bi-axial NiO AFM layer [15]. In this Letter, we propose a possible mechanism of spin current transfer through anisotropic AFM dielectrics, which may explain all the peculiarities of the experiments FIG. 1. Sketch of the model of spin current transfer through an AFM insulator based on the experiment [11]. The FM layer excites spin wave excitations in the AFM layer. The output spin current (at the AFM/Pt interface) is detected by the Pt layer through the inverse spin Hall effect (ISHE). [11, 12, 14]. Namely, we show that the spin current can be effectively carried by the driven evanescent spin wave excitations, having frequencies that are much lower than the frequency of the AFM resonance. We demonstrate that the angular momentum exchange between the spin subsystem and the AFM lattice plays a crucial role in this process, and may lead to the enhancement of the spin current inside the AFM layer. We consider a model of a simple AFM having two magnetic sublattices with the partial saturation magnetiza-tion M s. The distribution of the magnetizations of each sublattice can be described by the vectors M 1 and M 2 , |M 1 | = |M 1 | = M s. We use a conventional approach for describing the AFM dynamics by introducing the vectors of antiferromagnetism (l) and magnetism (m) [16–19]: l = (M 1 − M 2)/(2M s), m = (M 1 + M 2)/(2M s). (1) Assuming that all the magnetic fields are smaller then the exchange field H ex and neglecting the bias magnetic field, that is used to saturate the FM layer, the effective AFM Lagrangian can be written as [16, 18, 19]: